Illustrations of the Huttonian Theory of the Earth

by John Playfair

397. Though a local subsidence, or settling of the ground, could hardly account for this change, the pavement being perfect in its level, and the walls of the cathedral without any shake, yet a subsidence that has extended to a great tract, as to the whole of Italy, if the mass moved has continued parallel to itself, and changed its place slowly, will agree very well with the appearances. The facts here stated are also the more deserving of attention, that about Ravenna, the land, at the same time that it has sunk in its level, has extended its surface, and has encroached on the sea. Since the time of Augustus, the line of the coast has been carried farther out by about three miles.[204] This last is the undoubted effect of the degradation of the land by the rivers; and here we have very clear evidence of the forces, both under and above the surface, producing their respective effects at the same time, so that while the surface is raised by earth brought down by the rivers, every given point in the ground is depressed and let down to a lower level.[205]

[204] Manfredi, ibid.

[205] On the coast of Dalmatia also, the rising of the level of the sea has been remarked, particularly at the ruins of Diocletian's palace of Spalatro.

398. On the southern coast of Italy similar facts have been observed. Breislac, in his Topographia Fisica della Campagnia di Roma,[206] from certain appearances in the Gulfs of Baja and Naples, concludes, that at the beginning of the Christian era, the level of the sea was lower on that part of the coast than it is now. The facts which he mentions are the following: 1mo, The remains of an ancient road are now to be seen in the Gulf of Baja at a considerable distance from the land. 2do, Some ancient buildings belonging to Porto Giulio are at present covered by the sea. 3tio, Ten columns of granite at the foot of Monte Nuovo, which appear to have belonged to the Temple of the Nymphs, are also nearly covered by the sea. 4to, The pavement of the Temple of Serapis is now somewhat lower than the high water mark, though it cannot be supposed that this edifice when built was exposed to the inconvenience of having its floor frequently under water. 5to, The ruins of a palace, built by Tiberius in the island of Caprea, are now entirely covered by the sea.

[206] Cap. vi. p. 300.

Thus, it appears that the level of the sea is sinking in the more northern latitudes, and rising in the Mediterranean, and it is evident that this cannot happen by the motion of the sea itself. The parts of the ocean all communicating with one another, cannot rise in one place and fall in another; but, in order to maintain a level surface, must rise equally or fall equally over the whole of its extent. If, therefore, we place any confidence in the preceding observations, and they are certainly liable to no objection, either from their own nature or the character of the observers, we must consider it as demonstrated, that the relative change of level has proceeded from the elevation or depression of the land itself. This agrees well with the preceding theory, which holds, that our continents are subject to be acted upon by the expansive forces of the mineral regions; that by these forces they have been actually raised up, and are sustained by them in their present situation.

399. According to some other facts stated by the same ingenious author, it appears, that on the coast of Italy the progress of the sea in ascending, or of the land in descending, has not been uniform during the period above mentioned, but that different oscillations have taken place; so that, from about the beginning of the Christian era, till some time in the middle ages, the sea rose to be sixteen feet higher than at present, from which height it has descended till it became lower than it is now, and from that state of depression it is now rising again. Breislac infers this from two facts, which he combines very ingeniously with the preceding, viz. the remains of some ancient buildings, at the foot of Monte Nuovo, five or six feet above the present level of the sea, in which are found the shells of some of those little marine animals that eat into stone: And again, the marble columns of the temple of Serapis, which are also perforated by pholades, to the height of sixteen feet above the ground. All these changes Breislac ascribes to the motion of the sea itself; a supposition which, as we have seen, cannot possibly be admitted, since nothing can permanently affect the level of the sea in one place, which does not affect it in all places whatsoever.

400. Appearances, which indicate such alternations as have just been mentioned in the level of the sea, are to be met with on some other coasts. In England, on the coast of Lincolnshire, the remains of a forest have been observed, which are now entirely covered by the sea.[207] The submarine stratum which contains the remains of this forest, can be traced into the country to a great distance, and is found throughout all the fens of Lincolnshire. The stratum itself is about four feet thick; it is covered in some places by a bed of clay sixteen feet thick, and under it for twenty feet more is a bed of soft mud, like the scourings of a ditch, mixed with shells and silt.

[207] Phil. Trans. 1799, p. 145.

Here then we have a stratum which must have been once uppermost on the surface of the dry land, though one part of it is now immersed under the sea, and another covered with earth, to the depth of sixteen feet. A change of level in the sea itself will not explain these appearances: they can only be explained by supposing the whole tract of land to have subsided, which is the hypothesis adopted by the author of the description in the Transactions, M. Corria de Serra; the subsidence, however, is not here understood to arise from the mere yielding of some of the strata immediately underneath, but is conceived to be a part of that geological system of alternate depression and elevation of the surface, which probably extends to the whole mineral kingdom. To reconcile all the different facts, I should be tempted to think, that the forest which once covered Lincolnshire, was immersed under the sea by the subsidence of the land to a great depth, and at a period considerably remote; that when so immersed, it was covered over with the bed of clay which now lies on it, by deposition from the sea, and the washing down of earth from the land; that it has emerged from this great depth till a part of it has become dry land; but that it is now sinking again, if the tradition of the country deserves any credit, that the part of it in the sea is deeper under water at present than it was a few years ago. This might also serve to reconcile, in some measure, the phenomena of this submarine forest with the appearances which indicate an extension of the land on the coast of Lincolnshire. Indeed the extension of the land is no direct proof, either of its own elevation, or of the depression of the sea, as we may conclude from the instance of Ravenna already mentioned.